Hemp and Jointing Compound: The Narrow Modern Use Case for a Traditional Sealing Method
UK guide to plumber's hemp and jointing compound: when it's still appropriate, why it's banned on potable water, the right compound (Boss White, Hawk White, Water Hawk), and Loctite 55 as the modern alternative.
A homeowner walks into a plumbing merchant, tells the assistant they're replacing a stop valve on the rising main, and walks out with a roll of plumber's hemp and a tin of Boss White because the older man behind the counter said that's what their dad used. They wrap the threads, smear the paste, fit the valve, turn the water back on, and within a week have a quiet weep dripping into the under-stairs cupboard. The compound on that tin says non-potable only. The Water Fittings Regulations have prohibited hemp on drinking water since 1999. Nobody told them.
Hemp and jointing compound is a traditional sealing method that has not aged out of the trade entirely, but its legitimate use is now narrow and shrinking. Understand where it still belongs, and you'll know what to put on a new connection, what your plumber should be using on a stop tap, and why a tin labelled "natural gas" does not mean a homeowner can use it on a gas connection.
What it is and what it's for
Plumber's hemp is a coil of natural plant fibre, traditionally flax or cannabis hemp, used to pack the void between two threaded pipe fittings. It does the sealing job that PTFE tape does on small threads, but for situations where tape cannot fill the gap. The hemp is wrapped around the male thread of a fitting in the direction of tightening, and a paste known as jointing compound is smeared over the hemp before assembly. The hemp absorbs water once the system is filled, swelling slightly, which actively closes the residual void. The paste lubricates the joint during tightening and contributes to the seal.
The combination has a long pedigree. Iron pipework with threaded BSP (British Standard Pipe) connections was the dominant plumbing material in the UK for most of the twentieth century, and hemp with paste was the standard sealant. As copper pipe with capillary-soldered or compression fittings replaced iron, and as PTFE tape arrived as a tidy single-product alternative, the use case for hemp shrank. Two things have shrunk it further in the last 25 years.
First, the Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148) prohibit hemp as a jointing material on potable water systems. Schedule 2 of the regulations lists materials approved for contact with drinking water and hemp is explicitly excluded. Any compound or fibre touching water that will be drunk must be WRAS-approved (Water Regulations Advisory Scheme). Hemp is not. This applies to the entire mains-fed cold side and the hot supply that derives from it.
Second, hemp has been removed from IGEM/UP/2 (the Institution of Gas Engineers and Managers' standard for gas pipework in domestic premises) as an approved sealant. The sole surviving exception is the longscrew fitting with a backnut on steel pipe, which is trade work for a Gas Safe registered engineer, not something a homeowner touches. Industry articles still circulating online describe hemp as appropriate for gas; they are reflecting an older edition of the standard.
A tin of jointing compound labelled "suitable for natural gas" describes the compound in isolation. It does not give you permission to make a gas connection. All work on domestic gas pipework in the UK is legally restricted to Gas Safe registered engineers under the Gas Safety (Installation and Use) Regulations 1998. A homeowner who tightens a gas fitting is committing a criminal offence regardless of how good their hemp wrap is.
What's left, after those two restrictions, is a narrow band of legitimate homeowner use:
- Heating circuit connections (the non-potable side of the system: radiator valves, pumps, heating circulators, where the water is sealed inside the system and not consumed)
- Legacy galvanised iron pipework above 2-inch BSP, where you are repairing or extending an existing iron service and PTFE tape will not seal the coarse, often worn threads
- Large-bore valves and tap connections on threads of 1-inch BSP and above, where PTFE migrates under the high torque needed to seat a large fitting
For everything else, there is a better modern option. We'll come to that.
Hemp, paste, and what the paste actually does
The system is two products working together. Hemp without paste leaks. Paste without hemp works on small threads but cannot fill a large or worn thread void on its own.
The hemp packs the gap. The paste seals the spiral path that water would otherwise track along between thread crests. The paste also lubricates the joint as you tighten, allowing the threads to mate fully without galling. Most modern jointing compounds are non-setting, which means the joint can be undone in future without destroying the fitting. The hemp swells when wet and continues to seal even as the joint relaxes slightly through thermal cycling.
The paste you use matters. A lot of homeowners think any tin of jointing compound is the same. They aren't.
| Compound | Potable water | Heating circuit | Gas (Gas Safe trade only) | Approx 400g price | Notes |
|---|---|---|---|---|---|
| Boss White | No | Yes | Yes (compound only) | £5-£12 | BS 6956-5. The traditional white paste. Non-potable. Cheap and widely stocked. |
| Fernox Hawk White | No | Yes | Yes (compound only) | £8-£9.50 | Modern equivalent of Boss White. Non-potable heating and gas. Cleaner finish. |
| Fernox Water Hawk | Yes (WRAS approved) | Yes | Yes (compound only) | £10-£12 | The universal compound. WRAS approved for drinking water. Pay the small premium and you have one tin that does everything legally. |
| Flomasta Gas & Water | Yes | Yes | Yes (compound only) | £14-£15 (250g) | Soft-setting, high temperature/pressure rating. Mainly contractor use. |
The headline rule: if there is any chance the joint you are sealing is on the drinking water side of the system, buy Water Hawk. The £3 – £4 premium over Hawk White buys you a tin that's legal everywhere a homeowner can legitimately use jointing compound. Buying Boss White or Hawk White and applying it to a mains stop valve is a regulatory breach that may also be a slow long-term contamination risk depending on what's leaching from the paste.
Boss White is non-potable. The product is BS 6956-5 compliant for heating systems and gas (compound-only), but not WRAS approved for drinking water. If your merchant sells you Boss White for a mains water connection, they're wrong. Ask for Fernox Water Hawk specifically.
How to wrap and apply
This is the section that prevents the most common beginner failure. Direction is everything.
1. Clean the male thread. Wipe oil, debris, and any old jointing material off the threads with a clean rag. If you're working on a galvanised iron thread that has been previously made up, run a thread brush along it to remove fragments of old hemp. The thread crests need to be clean enough for hemp fibres to grip.
2. Tease out a length of hemp. Pull a strand from the dispenser or skein roughly twice the length of the thread circumference. You want a thin, even bundle. Most beginners use far too much: a thick rope of hemp prevents the fitting from threading on fully, and a half-engaged thread will not seal regardless of how much paste you add.
3. Wrap clockwise around the male thread. This is the critical step. Stand the fitting with the thread facing you and wrap the hemp clockwise. As you tighten the female fitting onto the male thread, it rotates clockwise too, which compresses the hemp into the void. Wrap anti-clockwise and tightening the joint will physically unwind the hemp out of the thread. Anti-clockwise wrap is the number one cause of hemp joint failure mentioned in plumbing forums and trade videos.
Lay the hemp into the thread valleys, building up to a thin even layer that just fills the depth of the thread cuts. The fitting must still thread on by hand for at least the first three or four turns. If it doesn't, you have used too much hemp.
4. Smear paste over the hemp. Take a fingertip's worth of jointing compound and work it into the wrapped hemp. The paste should coat every fibre but not be so thick that it squeezes out in great lumps as you tighten. A thin, even film is what you're after.
5. Assemble. Thread the female fitting on by hand until it stops easily. Then use a spanner or pipe wrench to tighten until the fitting feels firmly seated. There is no precise torque figure for hemp joints; experienced plumbers go by feel. The fitting should not move when you push it sideways. Excess hemp will protrude from the joint as you tighten; trim it flush with a sharp blade once the joint is made.
6. The quarter-turn advantage. This is what hemp does that PTFE cannot. If after assembly you find the fitting is pointing the wrong way, you can rotate it forward by a further quarter turn (or even three-quarters) without breaking the seal. Hemp's swelling and the soft compound accommodate the movement. PTFE-sealed joints lose their seal the moment you back the fitting off, even slightly. This is why hemp is still the trade choice on large valves where alignment matters: the gauge, the handle orientation, the pipe direction.
For a joint that needs precise alignment (a stopcock handle facing a specific way, a gauge readable from the front), make the joint slightly under-tight at first, then rotate forward to the correct position. With PTFE you must take the joint apart and reassemble. With hemp you finesse it in place.
When to use hemp instead of PTFE: the thread size rule
The trade rule, consistent across plumbing forums and the experienced plumbers who frequent them, is straightforward.
| Thread size / pipe | Sealant choice | Why |
|---|---|---|
| Up to 3/4-inch BSP (and 15mm/22mm copper compression backnuts) | PTFE tape | Tape fills small thread voids cleanly. Easy, fast, no mess. Use 6-8 wraps for water, gas-rated tape for gas connections. |
| 1-inch BSP and above | Hemp + compound | PTFE tears or migrates under the higher torque needed to seat a 1-inch+ thread. Hemp packs the larger void. |
| 2-inch+ galvanised iron | Hemp + compound, mandatory | Worn or coarse iron threads have voids that PTFE physically cannot fill. Hemp swells and seals. |
| Mains stop valve / appliance flexi tail (1/2" or 3/4" BSP) | PTFE on threads, fibre washer at the seat | Modern fittings have integral washers; the thread seal is secondary. PTFE is sufficient and standard. |
| Plastic-bodied fittings | PTFE only, hand-tight | Hemp and compound can over-stress plastic threads. The compound may also be incompatible with the plastic. |
The reason PTFE struggles on large coarse threads is mechanical. PTFE tape is thin and only seals when it is compressed evenly across the entire thread. On a 1-inch BSP or larger thread, the torque needed to seat the fitting is enough to tear the tape and force fragments out of the joint as you tighten. Hemp, by contrast, is a bulk fibre that compresses without tearing, and the paste acts as a sealing matrix around it.
A fitting on a 22mm compression backnut or a 1/2-inch tap connector does not need hemp. PTFE on those threads is faster, cleaner, and just as effective. Reserve hemp for the joints PTFE genuinely cannot do.
Loctite 55: the modern alternative
For most homeowners doing legitimate sealing work today, hemp and paste is no longer the right answer. Loctite 55 is.
Loctite 55 is a synthetic pipe sealing cord. It comes on a 150-metre or 160-metre roll in a small dispenser pack. The cord is wound clockwise around the male thread (same principle as hemp) but you don't apply any compound on top: the cord itself is impregnated with the sealing material. It's WRAS-approved for potable water, certified for natural gas (under BS EN 751-2), and rated for hot and cold water systems including central heating.
The advantages over hemp:
- Single product, no separate compound tin to buy
- WRAS approved everywhere a homeowner needs (potable water and heating)
- Cleaner application, no mess on the workbench
- Same quarter-turn adjustment advantage as hemp
- One roll seals dozens of joints
A roll of Loctite 55 (150m) costs £10-£15 at most UK merchants, which works out to a few pence per joint. The trade has been migrating to it steadily over the last decade, and several B&Q stores have been clearing hemp stock from shelves as the regulatory and product picture has moved on.
For a homeowner doing first-fix or second-fix plumbing on threads above 1-inch BSP, buy a roll of Loctite 55 instead of hemp and Water Hawk. You'll spend less, you'll have one product covering every legitimate use case (potable, heating, larger threads), and the application is cleaner. Reserve hemp specifically for legacy galvanised iron above 2 inches where a thicker fibre packing is genuinely required.
Cost and where to buy
Hemp itself is cheap. The compound costs more, and the right compound matters more than the hemp choice.
| Product | Pack size | Typical UK price | Notes |
|---|---|---|---|
| Plumber's hemp | 40g dispenser | £3.50-£5.50 | Trade Counter Direct, Toolstation, Screwfix when stocked. 80g and 100g balls also available. |
| Boss White | 400g tin | £5-£12 | BES (trade) cheapest, Toolstation consumer pricing higher. Non-potable only. |
| Fernox Hawk White | 400g tin | £8-£9.50 | Screwfix, BES, Toolstation. Non-potable heating and gas (compound). |
| Fernox Water Hawk | 400g tin | £10-£12 | Wickes, BES. WRAS approved. The right buy for any potable work. |
| Loctite 55 cord | 150m roll | £10-£15 | Screwfix, Toolstation. WRAS approved. Modern replacement for hemp+paste. |
UK trade and DIY merchants stock all of these. Screwfix, Toolstation, BES, and the larger plumbing merchants like Plumb Center and City Plumbing carry the full range. Wickes and B&Q stock Water Hawk and standard PTFE; their hemp and Boss White availability has declined as the regulatory picture has narrowed the homeowner use case.
If you are buying hemp, get the smallest dispenser size you can find. A 40g pack is enough for a dozen large joints; you do not need a 100g ball unless you're working through legacy galvanised pipework as a project. Hemp does not go off, but a half-used ball lurking in a damp shed eventually attracts moisture and discolours.
Common mistakes
Anti-clockwise wrap. Repeated for emphasis because it's the failure mode. Wrap clockwise on the male thread, in the same direction the female fitting tightens. Get the direction wrong and the joint will leak immediately, no matter how much hemp or paste you used.
Too much hemp. A fitting that won't thread on by hand has too much hemp on it. Half-engaged threads do not seal. The fitting needs to start by hand and finish on a wrench. Strip back to a thinner wrap.
No compound, just hemp. Hemp on its own does not seal. The fibre packs the void; the paste fills the spiral path. Both are needed. Forum posts from plumbers regularly point out homeowners who wrapped a thread with hemp, skipped the paste because they thought it was optional, and then puzzled over why the joint wept.
Wrong compound on potable water. Boss White and Hawk White are not WRAS approved. Using them on the mains side or on a hot water cylinder feed is a regulatory breach. Buy Water Hawk for anything potable, or use Loctite 55 cord which is approved across the board.
Hemp on push-fit or compression fittings. Hemp seals threaded joints. It does nothing useful on a push-fit, where the seal is an O-ring, or on a compression fitting, where the seal is the metal-to-metal compression of an olive. Wrapping hemp around an olive or shoving it into a push-fit prevents the actual seal from working. The same applies to flared and capillary connections.
Hemp on a gas connection. Hemp has been removed from IGEM/UP/2 as an approved sealant for domestic gas pipework. The longscrew exception is trade work, not homeowner work. All gas connections must be made by a Gas Safe registered engineer. A homeowner found to have made a gas connection (with any sealant) is committing a criminal offence and invalidating their home insurance.
Trusting older online articles. Several still-indexed UK plumbing blogs and product pages describe hemp as appropriate for domestic gas. They are out of date. Cross-check current IGEM standards or the Gas Safe Register before relying on online guidance for gas work, and remember that gas work is legally Gas Safe trade only regardless of what sealant guidance you find.
Where you'll need this
Hemp and jointing compound has a legitimate place at two stages of any extension or renovation involving plumbing, but the place is narrower than it used to be:
- First fix plumbing - large-bore threaded connections during installation: heating manifolds, large valves on the heating circuit, pump connections. Loctite 55 increasingly preferred for the same joints.
- Second fix plumbing - re-sealing or making good legacy galvanised iron pipework above 2 inches encountered during renovation, where PTFE cannot seal the worn threads.
These are non-potable, larger-thread situations on the heating side or on legacy iron services. For mains water, hot water cylinder feeds, tap connections, or anything that ends up at a drinking water outlet, use PTFE for small threads and Loctite 55 for larger ones. For gas, call a Gas Safe registered engineer.
The traditional hemp-and-paste system isn't dead. It's just been pushed into the corner of the trade where it still genuinely works better than the alternatives, and out of every other corner where the alternatives are now better, cleaner, and legally required.